When running the following one-liner, I get the mentioned error:
$ hg clone git+ssh://firstname.lastname@example.org/ehuelsmann/IPv6Usage.git $ cd IPv6Usage $ hg pull -B reimp283 abort: ref ea5314b26c3d23b5b85dcb36c166cf85fdc27087 not found on remote server
Working with JordiHG, we found that 'ea5314b26c3d23b5b85dcb36c166cf85fdc27087' is the hg hash, not the git hash. The git hash associated with the hg hash is 'e8ec30cfbb032c052ddf013aacb64cf3ae6563c9', which, according to JordiHG is the ref that belongs to the reimp283 bookmark.
(Note, I've provided a test for hg-git's test suite for this case a month or two ago)